Dispatch services, also known as Push-to-talk (PTT) or walkie-talkie services, provide near-instant, half-duplex communication between a dispatch caller and one or more target dispatch users. Dispatch services are typically offered as subscription services on wireless communications systems, which may also offer interconnect calling, short message service (SMS), packet data and other communications services to wireless subscribers. Each wireless communications system facilitates communications using one or more multiple-access wireless technologies such as code division multiple access (CDMA), global system for mobile communication (GSM) or time division multiple access (TDMA). Known dispatch technologies include Nextel's Direct Connect®, Qualcomm's QChat and Push-to-Talk over Cellular (PoC).
High performance dispatch networks are optimized to minimize communications latencies to achieve near-instant communications desired by dispatch subscribers. Such high performance networks typically include a regional dispatch application server that manages dispatch communications for local wireless subscriber units. To initiate and receive dispatch calls, the wireless subscriber registers with a regional location server and a dispatch application server servicing the network region. When the wireless subscriber roams to a different network (or region), it registers with the new network, which includes notifying the wireless subscriber's home network of its current location and receiving authorization to access local dispatch services.
Various frameworks exist for facilitating the provision of multimedia application services to roaming subscribers, including the IP Multimedia Subsystem (IMS) standard. IMS provides subscriber access to multimedia applications across disparate communications networks and administrative domains. In operation, a wireless subscriber roaming onto an IMS network registers once as an IMS client though the IMS network's Serving-Call Session Control Function (S-CSCF) to access IMS applications that are available through the S-CSCF. Conventional high performance dispatch networks, however, do not offer IMS dispatch application services through the S-CSCF, and require the wireless subscriber to separately register for dispatch and IMS services.